1. Field of the Invention
The present invention relates to a power supply for applying a high voltage and a deflection current to a cathode ray tube (CRT), etc.
2. Description of the Related Art
A high-voltage generation circuit using a flyback transformer has been employed in a power supply for a CRT. The high-voltage generation circuit using the flyback transformer supplies the anode voltage to an anode of the CRT and a deflection current to a deflection yoke of the CRT. A power supply enabling these two roles by a single flyback transformer is referred to as a deflection current/high voltage integration type power supply.
Conventionally, CRT displays for computers are required to display a highly precise image. Therefore, a deflection current/high voltage separation type power supply including a circuit part for generating a deflection current and a circuit part for generating a high voltage separately has been generally used. However, there arises a demand for a deflection current/high voltage integration type power supply for CRT displays for computers because of cost reduction and miniaturization requirements.
Such a deflection current/high voltage integration type power supply device controls the high voltage by feedback. However, the deflection current is affected and fluctuates due to the control for stabilization of the high voltage when the high voltage load changes. In general, the horizontal amplitude of the screen is proportional to I.sub.DY /(V.sub.II), where I.sub.DY is the deflection current and V.sub.II is the high voltage (anode voltage) to be output from the flyback transformer. Thus, the amplitude of the screen fluctuates according to the change in brightness of the screen.
To solve this problem, a power supply of a so-called diode modulator circuit system has been proposed. In this power supply, the high voltage is stabilized by controlling the primary input voltage (+B) of the flyback transformer and the deflection current is controlled by the bias to a transistor provided in a flow path of the deflection current.
However, in the power supply device of the diode modulator circuit system, there is a problem that a switch element for controlling the voltage +B mentioned above and an element for controlling the deflection current are also required in addition to a main switch element to connect/disconnect the primary current in the flyback transformer, and the number of total elements becomes large, and the circuit becomes complex. Further, there is a problem in that the responsiveness in stabilization of the high output voltage and stabilization of the deflection current deteriorate because smoothing circuits are interposed at the +B control part and the deflection current control part.